Hello Jenna and welcome to this site!
I plan to see Dr. Mark Rosengarten when I get pregnant as he is apparently one of the more knowledgable doctors in the Vancouver area in treating HG. He is located in the offices of the shopping centre across from the Vancouver City Hall. If you would like more info about him, let me know and I shall pass it along. A warning though: it takes an insane amount of time to get into see him. Of my two referrals to him, both have taken at least 3 months. And as for re-writing your US prescriptions I really don't know! I haven't been in that situation myself.
May I ask what you are taking now? If its Reglan or Phenegran you may not have any problem getting a rewrite. However if it's Zofran you probably will have some battles to get a doctor to write a prescription for you. Zofran (or Ondansetron as it is known here) is not as widely accepted as a treatment for HG as it is in the states.
Another option you may have is that there is an online pharmacy that fills some US prescriptions. There is a link somewhere on this site. I'll try and find it for you and post it.
Let me know if you have anymore questions.
Joanna